Chemistry

It's a gas

A gaseous metal may just have been discovered

IN CHEMISTRY, less is more. The subject's practitioners have a horror of what they call “bucket chemistry”—even if the buckets in question are merely test-tube sized. Finesse comes from the ability to analyse small quantities accurately. And the smallest possible quantity in chemistry is, of course, a single atom.

Robert Eichler, of the Paul Scherrer Institute in Switzerland, and his colleagues have not quite managed that, but they have come close. As they report in this week's Nature, they have characterised the properties of a recently discovered element using two atoms. In doing so, they have shown that the predictions of chemistry's central blueprint, Dmitri Mendeleev's periodic table, extend reliably into the realm of heavy, short-lived, radioactive elements that do not exist in nature. They may also have found the first metal that is a gas at room temperature.

An electronic shell game

As generations of chemistry teachers have laboured to convey, the periodic table consists of rows and columns. Adjacent elements in a row differ from one another by a proton in the nucleus and an electron orbiting that nucleus. The periodic table works because the electrons like to organise themselves in concentric shells. Outer shells are bigger than inner ones, but each has a fixed capacity. When a shell is full, the next element in the series starts a new shell, and that marks the beginning of a new row of the table. The larger capacity of outer shells means that the rows get longer, and so more columns appear.

It is the columns that are the interesting part of the table. Elements in the first column all have one electron in their outermost shell. Those in the second column have two outer electrons, and so on. Only electrons in the outer shell can form chemical bonds, so the elements in each column are chemically similar.

What Dr Eichler and his colleagues wanted to find out was whether the pattern would continue to hold good for the monster elements coming out of the Joint Institute for Nuclear Research in Dubna, Russia. The reason to suspect it might not is that as the shells get bigger, the velocity of the electrons within them increases. At some point they will be travelling so fast that the way they bond will be affected by Albert Einstein's theory of relativity, which departs from classical theories at speeds close to that of light.

The object of Dr Eichler's study was element number 112. This was first made in 1996, but has still not received an official name (it goes under the ugly moniker of ununbium). In non-relativistic theory, it should behave like the other members of its column—zinc, cadmium and mercury—and be a metal. But some relativistic calculations suggested it might actually behave like a different element, the inert (and non-metallic) gas, radon.

Unfortunately for chemists, ununbium can be made only a few atoms at a time (by bombarding plutonium atoms with calcium atoms), and those atoms that are made have a half-life of a mere four seconds. Dr Eichler nevertheless took up the challenge. With the aid of colleagues in Dubna, he managed to get two atoms of ununbium to stick to a sheet of gold. What is more, by using a trick called thermochromatography he was able to measure the energy released when this happened. All that remained was to see whether this energy release more resembled that of gold reacting with mercury or with radon.

Mendeleev 1: Einstein 0

By a convenient fluke, the process that produces ununbium also generates radon, while adding neodymium yields mercury, too, so Dr Eichler's comparisons were to hand. The upshot is that the ununbium/gold reaction resembled that with mercury and not that with radon. However, the interaction energy reveals more than just chemical affinity. It also gives an idea of how volatile an element is, even if only one or two atoms are available.

Whether an element is a solid, a liquid or a gas depends on how its atoms interact with each other. But how they interact with other substances, such as gold, acts as a proxy for that behaviour. Dr Eichler's previous work has established a graph that describes this proxy. Although he cannot be sure, there is a good chance that if you could make ununbium in large quantities, it would not only be metallic, but would also be a gas at room temperature.